The present invention is related to Field Programmable Gate Array (FPGA) integrated circuits and, more particularly, to antifuses in FPGAs and their manufacture.
Antifuses are found in a growing number of integrated circuits, most of which are FPGAs. As the name implies, antifuses have a very high resistance (to form essentially an open circuit) in the unprogrammed ("off") state, and a very low resistance (to form essentially a closed circuit) in the programmed ("on") state. In these integrated circuits antifuses are placed at the intersections of interconnection lines which lead to different elements of the integrated circuit. By programming selected antifuses, the interconnections between the various elements of the integrated circuit are formed to define the function of the device.
In a typical antifuse structure a programming layer of amorphous silicon is sandwiched between two metal interconnection lines. Depending upon the material of a metal interconnection line, a layer of barrier metal, such as TiW (titanium-tungsten), is used as an interfacing layer of the metal interconnection line to the programming layer. Barrier metal layers function to block the undesired interdiffusion of material from the programming layer, silicon from amorphous silicon, and material from a metal layer, aluminum from aluminum alloy, for instance. Barrier metal layers are typically refractory metals, their intermetallics, alloys, silicides, nitrides and combinations thereof.
An antifuse is programmed by placing a large voltage across the two metal interconnection lines. The programming voltage is much larger than the operating voltages in the FPGA and causes the programming layer to melt to form a conducting link between the two interconnection lines.
Nonetheless, various problems have been found with present day antifuses. One problem is that the programmed resistance (R.sub.ON) of the antifuses of the FPGA tends to vary widely and unpredictably, even to the point that the antifuse can be considered unprogrammed. This is undesirable because the failure of one antifuse may remove the defined function of the programmed circuit in the FPGA and the unpredictability of the resistances of the programmed antifuses also renders the performance of the FPGA to be inconsistent.
Another problem, which is believed to be related to the first problem, is that voltages to program the antifuses also tend to vary widely. Circuit and process designs must be made to accommodate these variations, which adversely affect the cost and performance of the integrated circuit incorporating these antifuses.
The present invention solves or substantially mitigates these problems. An advantage of the present invention is that the resulting antifuse has a low capacitance which allows a high performance of the FPGA. A further advantage is that the present invention can be incorporated into existing antifuse processes without radical and expensive changes.